Nanoelectronics

Small wonders

Traditional silicon memory chips are about to face a challenge

SMALL achievements, it seems, are big news these days. Within the past six months, both Hewlett-Packard (HP) and IBM have burst through the silicon ceiling. They have demonstrated circuitry based on radically new technologies that have a much higher density of components than any achieved by today's leading chip manufacturers. This opens up the possibility of storing gigabits of information in an area smaller than a fingernail. And although the two computer-makers' technologies are different, both point towards the end of the age of silicon.

At a meeting of Sweden's Royal Institute of Technology in Stockholm on September 9th, Stanley Williams, director of quantum science research at HP, announced that his group had created the highest density of electronically switchable memory so far. “Electronically switchable” means that each bit of memory can be flipped on and off by a current, without the need for any moving parts.

Earlier this year, an IBM project named Millipede demonstrated even higher potential storage densities. Millipede uses an array of tiny silicon cantilevers, with downward-pointing tips (“feet”) that punch indentations into a thin plastic film which moves beneath them. From a manufacturing perspective, though, Millipede may be less attractive than HP's new device, because it contains moving parts and is thus likely to require a higher degree of manufacturing accuracy.

HP's announcement was significant in three ways beyond the question of mere density. The memory bits are made of molecules known as rotaxanes, rather than silicon, making this the first significant demonstration of so-called molecular electronics. Second, it demonstrates the reliability of a new contact-printing technique that should allow molecular-electronic components to be printed cheaply. The third advantage is that HP has shown that such components can be read and “written to” in an economic way. Although the demonstration device contains only 64 bits, the tests suggest that scaling up should prove to be straightforward. One square centimetre of such molecular-electronic wizardry should therefore be able to store 6.4 gigabits of information.

The individual bits are not yet single molecules. Each consists of about 1,000 rotaxane molecules trapped in a minute junction between wires made of platinum and titanium. The wires themselves are only 40 nanometres (billionths of a metre) wide. Although HP is also looking at other metal/molecule systems, the wires are so tiny that it is not prohibitively costly to use precious metals.

The memory bits (ie, the ones and zeros of the binary arithmetic used by computers) are written into each cluster of molecules by changing the structure of those molecules. A small voltage is applied across the clusters using current in the metal wires. The resulting change in structure alters the electrical resistance of the molecules by a factor of 10,000—making it easy to measure whether the state of the cluster represents a “one” or a “zero”.

Researchers in the field do not yet understand the manoeuvrings that change the molecules' resistance. However, they do know that a phenomenon called quantum tunnelling is involved. During quantum tunnelling, one of many strange physical effects that occur at small scales, electrons spontaneously disappear from one place and reappear nearby.

The components of the circuitry are stamped on to the chip using an inexpensive technique called nano-imprint lithography. A master mould is carved out of silicon, using a precise beam of electrons. The resulting mould is pressed into a layer of polymer, and the trenches created in the polymer are filled with platinum. After that, a single layer of rotaxane is deposited on top and, once the mould has been rotated by 90°, another set of trenches and wires is made on top of the rotaxane.

Perhaps the most important part of HP's announcement is its demonstration of a “logic circuit” for addressing each bit of molecular memory. A logic circuit has several inputs and a single output. It is the physical embodiment of a mathematical equation and, as in an equation, the value of the output depends on the state of the inputs. The practical benefit of this is that any particular bit of memory can be addressed without having to measure the state of every single pair of wires.

One of the advantages of HP's device is that the memory is “non-volatile”. In other words, once the switches are flipped they stay that way. So memory will be retained even when the power is shut off. So far, HP's researchers have demonstrated that memory can be kept like this for four months. There is a snag. The switches typically work only 50-60 times before they start to degrade. On the other hand, as Dr Williams points out, with terabits of storage available, it may never be necessary to erase anything.

HP says that its molecular electronics will take a minimum of five years to commercialise. It intends to do it in part by publicly disclosing its results in order to “seed its own competition”. Initially, molecular electronics would be aimed at niche markets such as flash memory for cameras. Ultimately, though, it should compete with DRAM, the most common form of silicon-based memory

If HP or IBM (or Intel or Motorola, which are also working on molecular electronics) can commercialise their nano-scale devices, the memory-chip industry will leap ahead of predictions for its future based on silicon semiconductors. HP's demonstration has about ten times the density of memory of the best silicon DRAM memory on the market today. With the cost of new fabrication plants for silicon chips running to billions of dollars, a technology that is both better and cheaper than silicon-based memory can be expected to disrupt the status quo.